docs: def is_external(self):
          """Check if the device is external."""
          # NOTE: Checking for equality HintSystem==False returns False if the
          # property is resolved to a None value (interface not available).
          if self._P.Block.HintSystem == False:       # noqa: E712
              return True
          # NOTE: udisks2 seems to guess incorrectly in some cases. This
          # leads to HintSystem=True for unlocked devices. In order to show
          # the device anyway, it needs to be recursively checked if any
          # parent device is recognized as external.
          if self.is_luks_cleartext and self.luks_cleartext_slave.is_external:
              return True
          if self.is_partition and self.partition_slave.is_external:
              return True
          return False